(a) Field of the Invention
The present invention relates to current sources. More specifically, the present invention relates to a temperature-independent current source circuits.
(b) Description of the Related Art
A voltage source and a current source are essential circuit components in analog circuits. Voltage and current sources should generate substantially constant voltage and current even if surrounding environmental factors change. For example, voltage and current sources should not influenced by the variations of the load and the temperature, thus enabling a stable operation of the system. In particular, essentially temperature-independent currents should be supplied to elements, which may be sensitive to temperature variations, such as transistors.
Exisiting circuits are illustrated in FIGS. 1(a) and 1(b), showing a PTAT (proportional to absolute temperature) current source and an NTAT (inversely proportional to absolute temperature) current source.
Referring to FIG. 1(a), a PTAT source includes two transistors Q1 and Q2, a resistor R1, and a current mirror. The collector currents of transistors Q1 and Q2 are essentially the same when the current ratio of the current mirror is equal to one. Values of the collector currents are given:
                    IPTAT        =                                            VT                              R                ⁢                                                                  ⁢                1                                      ⁢            ln            ⁢                                                  ⁢            N                    =                                                    k                ·                T                                                              q                  ·                  R                                ⁢                                                                  ⁢                1                                      ⁢            ln            ⁢                                                  ⁢            N                                              (        1        )            
where IPTAT is a value that corresponds to the collector current of the transistor, VT is a thermal voltage: VT=kT/q with a value of about 25 mV at room temperature (the room temperature is 27° C., corresponding to an absolute temperature of 300K), N is a ratio of the emitter area of the transistors Q1 and Q2, q is the absolute value of the charge of an electron, k is Boltzmann's constant, and T is the absolute temperature. A possible realization is illustrated in FIG. 1(a), representing a current source that outputs a current that is substantially proportional to the absolute temperature since IPTAT is proportional to the absolute temperature T.
Referring to FIG. 1(b), a current source that outputs a current that is inversely proportional to the absolute temperature T includes two transistors Q3 and Q4, a resistor R2, and a current mirror 10. Current mirror 10 of FIG. 1(b) has the same function as that of the current mirror of FIG. 1(a). Since the ratio of the input and output of the current mirror is essentially one, the values of the collector currents of transistors Q3 and Q4 are the same. This collector current value is determined by transistor Q3 and resistor R2:
                    INTAT        =                  VBE                      R            ⁢                                                  ⁢            2                                              (        2        )            
where INTAT is the collector current of transistors Q3 and Q4 and VBE is a base-emitter voltage of transistor Q3, which can be a bipolar transistor. Since VBE decreases as the temperature increases, VBE is reduced by about −2 mV when a junction temperature is increased by about 1 degree. Accordingly, the circuit of FIG. 1(b) is an NTAT current source as described by Equation (2).
As described by Equations (1) and (2), the current sources of FIGS. 1(a) and 1(b) are influenced by the temperature.
Temperature-independent current sources have been created in the past by combining a PTAT current source and an NTAT current source, as described in U.S. Pat. Nos. 6,310,510 and 6,023,185. U.S. Pat. No. 6,310,510 described a circuit functioning as an NTAT current source and a circuit functioning as a PTAT current source, and combined them into a temperature-independent current source, thereby requiring a lot of circuit elements, increasing the cost. This architecture also lowers the quality of the current source because of a problem of matching both circuits. This matching problem leads to an increased sensitivity of the output current to the temperature. Further, U.S. Pat. No. 6,023,185 requires a band gap reference.